I was going through the settings. I changed the resolution from 800 x 600 to 1280 x 600 and then the screen went black, I tried to change it back but I couldn't. Now whenever I run it (even after deleting and reinstalling the game), I just get a black screen. How do I change the settings back to default?

go to library -> right click super meat boy -> properties -> general -> set launch options... -> copy and paste this into it -> -windowed -1920x1080 -highdetail <- change the details to -lowdetail or -mediumdetail if required and it should be fine